to insure that no one sits in your seat for five minutes if you get up (to throw out trash, go to the bathroom, grab some chips, etc.)
it literally means that you call your seat for five minutes; if you call fives, get up, and come back ten minutes later, your seat is liable to be taken
a very good system to avoid fights, bad feelings, and general undesirable situations
there is no such thing as 'calling tens' or any number other than five; this is because five minutes is a reasonable amount of time to be gone for

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